The present invention relates to a polyphase, preferably a three-phase, a.c. stator system for a linear motor.
Linear motors are well known generally and for a considerable length of time; also, linear motors have been constructed in a variety of configurations; there exist d.c. motors, and synchronous as well as asynchronous a.c. motors. Generally speaking, a linear motor includes a stator being arranged in, or along, a line but not in an annular configuration. The moving armature is correspondingly constructed, it does not rotate.
The linear motor provides, generally, a conversion of electrical energy into translatory, mechanical energy. A three-phase motor, for example, has three separate energizing systems, being suitably arranged in the stator body and inductor. The armature may consist of a rail element made of copper or aluminum, i.e., of a good electrical conductor in order to establish an asynchronous motor. Alternatively, the armature may be comprised of a permanent magnetic body for establishing a synchronous motor. A certain variety of linear motors include coils on the armature.
Linear motors are used for a variety of purposes, such as "people movers," as a drive in the conveyor and transport art, as, for example, for handling baggage or, generally, for moving freight. Linear motors are also used in mining, for operating cranes, drag equipment, slides in machine tools, for the operation of gates, and so forth. The length of such a motor depends greatly upon its use and the length of the path to be traversed.
A linear motor of the type to which the invention pertains is usually comprised of an elongated inductor body having grooves for receiving windings, and it cooperates with the armature which is caused to move along the inductor stator. The stator windings are, for example, comprised of cables suitably placed into the grooves. For instance, it is known to construct a coherent, ladder-like assembly from three cables which are interconnected in such a manner that they constitute a prefabricated entity, can be transported as a whole, and just be laid into the stator grooves. The spacing of the "rungs" of the "ladder" corresponds to the groove-spacing of the stator and inductor body which has previously been installed along the path that the armature is to take (see, e.g., U.S. Pat. No. 4,246,694, Ser. No. 909,794, filed on May 26, 1978).
If such a linear motor is to be used, for example, as a motive power unit for a magneto-aerial railway, the stator is inherently very long. Since the motor will be operated at a rather high voltage, the cable must be provided with at least one shield. The shield for such a cable is needed for a variety of reasons. Capacitive charges have to be conducted at not too high a resistance; short circuits of the cables to ground and other faults have to be recognized and located via such a shield, and the cable insulation proper must be protected against damage. Furthermore, the shield should protect people and animals against shocks from the rather high voltage, even though the voltages are usually "only" in the median range, such as 1.500 V. It is apparent that the shield fulfills its function the better, the closer its potential is to ground potential. Ideally, the entire shield is grounded.
Such a cable, having an external shield and being used for establishing stator windings of a linear motor, may experience longitudinal voltages in the order of 1 kilovolt per 100-meter stator length. Such a high voltage can be suppressed, e.g., by subdividing the shield into much smaller sections and grounding each section separately. This obviously is a very expensive approach and, besides, cable defects would readily be invited.
Another approach would be to simply ground the shield at both ends of a lengthy section which would, however, merely reduce the effective voltage in and along the shield. Also, one can interconnect, electrically, the shield of the various (cable) phases in various points along the stator. However, this approach entails high shield currents with correspondingly high losses of electrical energy. The shield's system may actually function as a kind of eddy current brake.
Test construction involve one-sided grounding of the shield in points spaced at large distances from each other. But that approach requires careful insulation of the shield itself because by and in itself, it does not function as a protection for people or animals.